Cytokinin inhibition of respiration in mitochondria from six plant species.

The influence of 6-benzylaminopurine (BAP) on the respiration by mitochondria from bush bean (Phaseolus vulgaris L.), mung bean (P. aureus Roxburgh), soybean [Glycine max (L.) Merrill], maize (Zea mays L.), pea (Pisum sativum L.), and wheat (Triticum aestivum L.) was examined. BAP, a synthetic cytokinin, consistently inhibited oxygen uptake by mitochondria from all species when malate was used as the substrate. The decrease in respiration was especially evident in the presence of ADP or an uncoupler of oxidative phosphorylation. 6-Isopentenylaminopurine and 6-furfurylaminopurine also inhibited malate oxidation, but zeatin and adenine did not. In certain instances, BAP reduced succinate and NADH oxidation. With succinate as the substrate and with antimycin A present, inhibition by BAP paralleled that caused by salicylhydroxamic acid, an inhibitor of alternative respiration. A suggested scheme features a cytokinin-inhibited point located between NADH dehydrogenase and cytochrome b of the electron transport system. Electrons from the NADH generated by malate oxidation are assumed to flow through this point, with electrons from externally supplied or cytosolic NADH and succinate doing so only under certain conditions such as when alternative respiration is occurring. Cytokinin effects on respiration and perhaps on other phenomena may be mediated by this mechanism.